The world operates currently through continuous depletion of basic utilities such as energy and freshwater, and sees an ever-increasing cost of raw materials. Thus, it has become increasingly important to improve the sustainability and efficiency of processes of fine chemical and pharmaceutical synthesis. One solution, which enables fewer reagents, less waste materials, high throughput, more efficiency, increased safety and reduced environmental impact, is represented by the use of continuous, small-dimension flow reactors. The use of such continuous-flow devices avoids the drawbacks associated to either conventional “batch” synthesis or scale-up when moving from laboratory to market-size production.
Continuous flow micro-/milli-reactors (reactors having working fluid passage cross-sectional dimensions in the micro- to several milli-meter range) have demonstrated the ability to increase process efficiency due to the intensification of heat and mass transfer processes. The effects on the chemical reactions are beneficial, enabling the reactions to be operated in desirable windows of temperature(s) and concentration(s), thus decreasing the generation of by-products. Furthermore, due to the small in-process volume of continuous flow reactors and their higher controllability, relative to batch reactors, the hazards associated with dangerous chemical processes are considerably reduced.
It is desirable that continuous flow micro-/milli-reactors be thermally stable and very chemically resistant in order to be compatible with the widest possible range of chemicals and reactions. Some ceramic materials—for example, alumina, silicon carbide and aluminium nitride—possess excellent thermal stability and relatively high resistance to a wide range of chemicals. Silicon carbide is especially chemically resistant. Some of these ceramic materials also have relatively high thermal conductivity, as high as some metals, which can be an advantage where high heat transfer rates are needed in the reactor.
High thermal conductivity of the flow reactor can also be a disadvantage however. This is because it is desirable to have as much passage length and passage volume as possible within a given reactor module, which makes it desirable to use a process passage with a folded geometry, while at the same time folded passages are at risk of thermal cross talk, in which a large amount of heat is produced (or absorbed) at one location along the process passage, and the heat spreads (or the cold spot spreads) to other positions along the process passage, with detrimental effects. It would be desirable to be able to use in a flow reactor module a highly chemically resistant material which is also relatively highly thermally conductive, while avoiding or significantly reducing thermal cross talk.